1. Field of the Invention
The present invention relates to an actuator having a wave gear reduction drive, particularly to an actuator having a structure that enables the axial length of the actuator to be reduced.
2. Description of the Prior Art
Actuators include a motor and reduction gear and are configured to reduce the speed of the motor rotation by a prescribed reduction ratio to effect linear or rotational movement of a load. Normally the motor rotation is transmitted to the reduction gear by coupling the motor output shaft to the input shaft of the reduction gear. A wave gear reduction drive is a reduction gear that has few parts and is capable of providing a high reduction ratio. A wave gear reduction drive is comprised of three components, which are a rigid, circular internal gear, a flexible, circular external gear, and a wave generator that constitutes the input element. Thus, in the case of an actuator equipped with a wave gear reduction drive, the motor shaft is connected to the wave generator by a coupling.
An actuator thus equipped with a wave gear reduction drive tends to have a long axis, since the wave gear reduction drive is disposed in the axial direction of the actuator.
An object of the invention is therefore to provide a configuration for an actuator having a wave gear reduction drive that enables the length of the axis thereof to be decreased.
In accordance with the present invention, the above object is attained by an actuator comprising a housing and a motor and wave gear reduction drive disposed adjacently within the housing along the center axis of the housing, the wave gear reduction drive including a circular, rigid internal gear, a circular, flexible external gear that is capable of radial elastic displacement, and a wave generator that radially displaces the flexible external gear into partial engagement with the rigid internal gear while circumferentially rotating points of the partial engagement, the flexible external gear including a cylindrical body portion that is capable of radial elastic displacement, an annular diaphragm that extends radially inward or outward from an end of the body portion, and a boss formed as a continuous part of an inner or outer edge of the diaphragm, the wave generator including a rigid cam plate and a bearing with internal and outer rings capable of radial elastic displacement disposed on the peripheral surface of the cam plate, with the cam plate being driven to rotate by the motor, the motor having a rotational shaft that includes a motor shaft portion to which a rotor is attached and an extended shaft portion that extends from an end of the motor shaft portion towards the wave gear reduction drive, the wave generator cam plate being formed integrally on the peripheral surface of the extended shaft portion, and, sandwiching the portion at which the cam plate is integrally formed, the motor shaft portion of the rotational shaft being rotatably supported in the housing via a first bearing, and the wave gear reduction drive end of the rotational shaft being rotatably supported by the flexible external gear boss via a second bearing.
In the actuator of this invention, the motor shaft and the cam plate are formed from the rotational shaft, which is a single component, and the rotational shaft is supported by first and second bearings. The first bearing can be a typical support type with a fixed outer ring and a rotating inner ring, while in the case of the second bearing the outer ring is affixed to the boss of the flexible external gear that is the rotational output element of the actuator, and the inner ring is affixed to the rotational shaft extension portion, so that both rings are supported in a state of differential rotation.
An actuator thus configured in accordance with the present invention can be made with a shorter axial length than a conventional actuator in which the motor, coupling and reduction gear are all connected in the axial direction.
In a typical actuator, an internal partition separates the housing into the space where the motor is located and the space where the wave gear reduction drive is located. In such a case, the inside face of a central opening formed in the partition can be used to rotatably support the rotating shaft via the first bearing. In this case, a single member can be used to integrally form the rigid internal gear and the partition. This would eliminate the need to use screws to assemble the two parts, and by eliminating the step of joining the two parts together, would make it possible to decrease the number of assembly steps required. This would also ensure enough space for the motor""s windings.
In a typical wave gear reduction drive, the flexible external gear has the annular diaphragm that extends radially inward from the end of the body portion, and a cup-shaped boss formed as a continuous part of the inner edge of the diaphragm. In this case, the second bearing can be supported by an annular bearing seat attached to the boss. Moreover, a typical motor has an encoder attached to the motor shaft, with the motor being controlled based on the encoder output. In this case, the rotational shaft can be provided with a second extended shaft portion, formed to extend from the other end of the motor shaft portion, and the encoder can be attached to the second extended shaft portion. A hollow actuator can be formed by using a hollow rotational shaft and providing the flexible external gear boss with a through-hole that is concentric with the hollow shaft.